How can I control the intensity of the light when using volumetric scattering?
I agree with some of the things you want to add.


add the white (or black, or grey) chimney smoke. I think that would make it cooler and a tad bit more realistic.
fix UV
smog is cool, but first find a reference image of what you want the smog to look like from above (very very last bullet below... keep reading)


However:


don't add trees, (unless you can make them almost perfectly indistinguishable from a real life tree)
don't use the first image


Things you need to work on:


Add more detail to the building right in front of the camera. This is an immediate giveaway that your scene looks fake. find whatever building you were trying to model and make sure that your building looks just like it from up close.
A lot of your textures aren't great. The building in the bottom center has a really depressing texture so when you look at it, it looks like the building is falling apart or something (if that was your goal, its not a bad texture). Make sure to bump map all your textures (but don't overdo it), and make all your windows at glossy so that your houses don't look like they are made of paper.
Rather than changing the sun's settings, try moving it around some. If the scene is back lit, you won't have to worry as much about the textures because you can do some post processing to make your buildings silhouettes.
Try using depth of field, (maybe just a little bit) so that the building closest is a little bit out of focus. This will reduce its sharpness so that you don't have to make it as realistic.
And finally, find a reference image so that you know what you want, and can gauge your success.


Besides all of these, I quite like your composition. Keep it up!